Negative-electron-affinity materials, such as monolayer coatings of cesium on GaP, Si, etc., and thin oxide layers on metallic substrates have proven to be excellent secondary-electron emitters for low-power applications such as electron multipliers. There has been less success, however, in finding good secondary emitters for higher-power devices, where heating and electron or ion bombardment can destroy the surface layers or coatings.
Finely-grained two-component films comprising a high yield material and an electrically conductive material are good candidates for use as secondary-electron emitters, especially in high-power applications.
Briefly, metal oxides such as BeO, MgO and Al.sub.2 O.sub.3 exhibit some of the highest secondary-electron yields. However, their usefulness as cold cathodes is limited to layers thin enough (usually &lt; 100A) for charge to tunnel from a conducting substrate to neutralize the surface charge produced by the primary electron beam. Such thin layers are unsuitable for high-power applications, in which they would be rapidly damaged by electron or ion bombardment. An alternative approach is to use finely-grained, two-component films comprising a high-yield material and an electrically conductive material. (This type of material is one of a class of materials usually referred to as cermets, and herein we shall refer to it simply as a cermet.) If the grains are small enough (about 100A or less), charge can tunnel or leak from the surface of an insulating grain to adjacent metal particles and then be conducted to a metallic substrate, even for relatively thick films. One such cermet system, Al.sub.2 O.sub.3 /Mo, has been investigated previously as a secondary-electron emitter, but problems were encountered with multiple chemical phases, and its secondary-electron yield was only four with a crossover energy no lower than 40 volts. The phenomenon of differential sputtering to be described below, was not investigated in this material.
It is therefore an object of this invention to provide a high-efficiency secondary-emitter which will be useful in devices where heating or electron- or ion-bombardment rapidly destroy conventional emitters.
It is therefore another object of this invention to provide thick film cermets having long life, high-efficiency secondary-electron emission and low crossover energy.